The aging 18,600-square-foot building might have five more years of useful life, city officials say. Perhaps 20.

But with a Nov. 5 referendum looming on whether to tear down the three-story building to make way for a new $160.5 million Clearwater Marine Aquarium, opponents are saying the city should investigate how much it would cost to patch up City Hall, which houses only 55 of the city's 1,520 employees.

Voters have the right to know how much it would cost to keep the doors open at 112 S Osceola Ave., they say.

"The city is not doing its due diligence. It's not doing its obligation," said Joe Corvino, who has been an active opponent of CMA's proposal, which includes paying the city $7.5 million for a new city hall.

The city hasn't done a study assessing how much repairing and maintaining the 47-year-old building would cost, but the bill would be more painful with every passing year, said city engineer Mike Quillen.

"We can keep patching City Hall together to get a few more years out of it, but sooner or later we face some pretty significant costs," Quillen said.

Aside from the plumbing and wiring that are nearing the end of their "useful life," the heating and cooling system is on its last legs, according to Quillen. And gutting the building to repair those problems would mean dealing with asbestos, an expensive dilemma, he said.

Whether or not City Hall should be repaired isn't the issue, city leaders say, but whether City Hall is in the right location.

Since 2000, when a waterfront redevelopment referendum was defeated, business and community leaders have agreed that City Hall shouldn't occupy such a prime spot, said City Manager Bill Horne.

"The feeling was, there are other places you could have a city hall that would serve its purpose than to have it with this kind of a vista of the Intracoastal and what this bluff area provides," Horne said recently, gesturing out of his office window to a expanse of blue water winking in the sun and dotted with boats.

Vice Mayor Paul Gibson finds CMA's offer appealing. The City Hall property was recently appraised at $6.6 million. CMA, which will only lease the property, not buy it, has agreed to give the city $7.5 million toward construction of a new city hall — a sum it will raise over time through a 50-cent surcharge on aquarium ticket sales — and to pay interest on the amount. Once the $7.5 million has been paid, CMA will be charged $250,000 a year through the end of the 60-year lease.

"I wish all my decisions were this easy," Gibson said.

Corvino called that math "insane," pointing to CMA's "break-even" attendance estimates of 975,000 as too optimistic. If aquarium attendance falls short, the lease payments may never be made, he said.

That's one reason why the city needs to figure out how much it costs to patch up City Hall, he said.

If the referendum passes, city leaders say they have several city-owned properties where they could consider building a new city hall, including the site of a soon-to-be-demolished downtown fire station, the property where the former Tampa Bay Times Clearwaterbureau was located, and parcels near the Municipal Services Building, where most of the city staff works.

No decision has been made about the size of a new city hall, Horne said, but it could possibly be a smaller, more energy-efficient version of what exists now, housing just the offices of the City Council, the city manager's staff and the city attorney.

A new city hall close to where most city employees work — in the Municipal Services Building at 100 S Myrtle Ave. — would make sense, Gibson said.

"City Hall is isolated by itself down by the bridge," he said.

Charlie Frago can be reached at cfrago@tampabay.com or (727) 445-4159. You can follow him on Twitter @CharlieFrago. To write a letter to the editor, visit tampabay.com/letters.